The present invention generally relates to wireless communication systems. In particular, the present invention relates to co-located omnidirectional and sectorized base stations providing increased capacity, redundancy and spatial diversity in a base station within a wireless communication system.
In a cellular system, a base station is used in each cell to provide the link between subscriber units within the cell and the rest of the communications network. One type of base station provides omnidirectional coverage. Because the antenna of an omnidirectional base station transmits and receives signals equally well in all directions, signals emanating from the base station may interfere with signals in surrounding cells in all directions. Thus, frequency reuse within the entire wireless communications network must be limited in order to ensure reliable communications. Limited frequency reuse within the cellular system results in decreased system capacity, and thus lower revenues may result. A typical frequency reuse rate in an omnidirectional base station is every seventh or twelfth frequency.
To overcome the problem of limited frequency reuse, base stations configured to provide coverage to a number of sectors radially distributed around the base station were developed. These base stations (known as sectorized base stations) allot available frequencies to each sector in such a way that frequency reuse throughout the cellular network is increased. Because the coverage area of the base station is subdivided into sectors, each of which is supported by a separate directional antenna aligned with it, signal interference with surrounding cells is reduced. A typical frequency reuse rate in a sectorized base station is every fourth or seventh frequency.
However, sectorized base stations also have drawbacks. Because the frequencies are allotted between the multiple sectors, subscriber units within a given sector can only use a fraction of the channels available to the entire base station, which reduces the trunking efficiency of that cell. For example, if an omnidirectional base station with 60 channels were replaced with a sectorized base station having three sectors, the sectorized base station could only provide 20 channels to subscribers located in any one of the sectors (ignoring for a moment the better frequency reuse). With a 1% blocking probability, these 20 channels can support 12 Erlangs of traffic in each sector, and 36 Erlangs in the entire base station. On the other hand, an omnidirectional base station could provide a trunking pool of 60 channels to subscribers no matter where they are within the cell. These channels could support 46.9 Erlangs of traffic at the same blocking rate. Thus, a portion of the capacity gain achieved by improving the frequency reuse through sectorization is lost due to a decrease in trunking efficiency.
Another problem encountered with both omnidirectional and sectorized base stations is the cost of combating the occurrence of multipath fades. Multipath fading refers to signal attenuation caused when multiple propagation paths of different lengths exist for the signals traveling between the base station and a subscriber unit. As these multiple signals arrive at the receiver of either a subscriber unit or the base station, they may be out of phase with each other. When they are combined to form the received communications signal, constructive or destructive addition may occur. If destructive addition results, the resulting communications signal may be greatly attenuated.
For the reverse link (subscriber unit to base station), spatial diversity is often used to combat multipath fading. Spatial diversity can be achieved when multiple atennas are available at the base station, allowing for a choice of signal source. However, this requires not only additional antenna hardware, but also signal processing hardware to effectively combine the signals from the multiple antennas. This is costly. On the forward link (base station to subscribe unit), spatial diversity is impractical due to the size limitations of the mobile subscriber unit, and therefore higher transmit power is needed from the cell site. This too is costly. In the case of a stationary or fixed subscriber unit, multiple antennas could be deployed, but cost again becomes an issue.
Yet another problem to address with base stations is the need for redundancy. A malfunction in equipment at the base station may lead to a loss of wireless communications service within the cell covered by an omnidirectional base station. In the case of a cell covered by a sectorized base station, a malfunction may cause loss of service in one sector or the entire cell. Thus, it was possible for equipment failures to cause hundreds of subscriber units to be without service. Redundancy is typically provided by redundant CPUs, redundant trunk lines, spare channel cards and spare antennas.
The presence of these and other problems in past system demonstrates that a need has long existed for an integrated approach to base station design that provides increased capacity, spatial diversity and redundancy within a cell of a wireless communications system.